Portable electronic devices such as IC cards, mobile phones, digital cameras, etc. are widely used. In recent years, there has been demand for all of these devices to become thinner, smaller, and lighter without exception.
In order to meet this demand, semiconductor chips used in these devices must also become thinner.
It has been said that the thickness of silicon wafers (simply referred to as a wafer or wafer chips hereinafter) used to produce semiconductor chips must be reduced to between 25 μm and 50 μm for the next-generation semiconductor chips, while the current wafer thickness is between 125 μm and 150 μm.
In the conventional methods of thinning wafers, protection tape is applied on the circuit formed surface of a wafer, the wafer is inverted, and the bottom surface of the wafer is ground with a grinder to become thinner. Thereafter, the bottom surface of the wafer is polished.
The bottom surface of the thinned wafer is seated on dicing tape held by a dicing frame, and the protection tape is peeled from the circuit formed surface of the wafer. Thereafter, the wafer is diced into chips using a dicing device.
However, in this method, breaks or cracks off are often made in a wafer when protection tape is peeled from the wafer. Further, a thinned wafer cannot be supported only on protection tape.
This requires that there be careful manual labor for transportation of such a wafer. Accordingly, processing steps cannot be automated.
In order to cope with this situation, a protection substrate obtained by impregnating a ladder type silicon oligomer to a stomatal sintered body of aluminum nitride-boron nitride is sometimes used instead of protection tape in order to hold wafers. In other cases, a protection substrate (support plate) consisting of alumina, alumina nitride, boron nitride, silicon carbide or the like, which have a thermal expansion rate substantially the same as that of the wafers, is used to hold the wafers.
In such cases, a wafer and a protection substrate have to be bonded together. For an adhesive material thermoplastic resin such as polyimide is used, specifically, a film of the thermoplastic resin with a thickness of between 10 μm and 100 mμ, or a film obtained by spin-coating adhesive resin solution and drying it with a thickness equal to or smaller than 20 mμ is used.
The above method, which uses a hard protection substrate, makes it possible to automate the handling and transport of wafers during processing steps.
However, this method uses a thermoplastic film that has been dried as an adhesive agent for bonding a protection substrate and a thermoplastic film, and thus a heating step has to be performed in order to soften the thermoplastic film when releasing the wafer from the protection substrate.
Softening a thermoplastic film that has been dried requires a high temperature, posing a high risk that circuits on a wafer may be damaged.
Further, the use of film adhesive causes a variation among portions in bonding strength so that some portions may be peeled when being polished and other portions may be difficult to be released when a wafer is released from the protection substrate, which are problematic.
In order to overcome this problem, Japanese Patent Application Publication No. 2005-191550 (See Abstract and FIG. 3) proposes a method in which a wafer is attached by using an adhesive agent to a support plate consisting of a rigid body with a number of through holes before the processes of grinding/polishing a wafer to a thinner state are performed in order to reduce the probability of making cracks in the wafers.
This method is advantageous in that an adhesive agent can be used highly efficiently because there are a number of through holes in the support plate, in that wafers are easy to handle even after being ground/polished into an extremely thin state because the support plate is a rigid plate made of the glass, or the like, and in that a release agent can easily penetrate an adhesive agent via the through holes provided to the support plate.
When a wafer is to be ground and polished, the bottom surface of the support plate holding the wafer is vacuum suctioned to be fixed to the processing stand.
If the adhesive agent layer is soft, the pattern of the through holes can be transferred to the circuit formation surface of a wafer because there are a number of through holes in the support plate as described above.
Also, the pressure from a grinding stone can sometimes cause the transfer of the pattern of through holes to a wafer in reality.
In order to prevent such transfers, anti-transfer sheets are used; however, the use of such sheets does not prevent the transfer of the through-hole pattern completely.
This problem can be solved by using harder adhesive agent layers. However, too hard an adhesive agent layer can cause a decrease in the bonding property, allowing the periphery of the wafer that has been thinned through grinding and polishing to be released and peeled from the adhesive agent layer.
Also, to a support plate, an adhesive agent layer is not applied but is adhered in a compression bonding method. This causes a problem wherein the bonding property of an adhesive agent layer that is too hard is low.
Accordingly, the thickness, material, and hardness, etc. of an adhesive agent have to be sufficiently adjusted in accordance with the thickness of the polished wafer, which is troublesome.